To ensure accessibility and availability of mental health services in the community and to reduce the burden associated with depression and related conditions, the study aim to evaluate the effectiveness of a collaborative stepped-care intervention (cognitive behavioural therapy-based) for adults with subthreshold depressive symptoms at the district-based primary healthcare setting.

Depression is among the most prevalent mental health conditions globally and contributes substantially to the global burden of disease. In adults, depression is highly comorbid not only with other mental health conditions (e.g., anxiety) but also physical diseases.

Since 2016, the research team has developed a collaborative stepped-care model for older people at-risk of or with depression in Hong Kong through the Jockey Club Holistic Support Project for Elderly Mental Wellness (JC JoyAge) (Clinical Trials Identifier: NCT03593889); the effectiveness and cost-effectiveness of which has been demonstrated, thereby leading to its implementation in social care settings across all 18 districts of Hong Kong.

To further improve the accessibility and availability of mental health services in the community and to reduce the burden associated with depression and related conditions, the JoyAge collaborative stepped-care intervention (cognitive behavioural therapy-based) will be expanded to target adults aged 45 years and above at the district-based primary care setting. This study thus sought to evaluate the effectiveness of this expanded intervention model.

Objective: To examine the effectiveness of the collaborative stepped-care intervention for adults with subthreshold depressive symptoms, as compared with propensity score-matched controls, in reducing depressive symptoms (primary outcome) both post-intervention (T1) and for up to 3 months of follow-up (T2). Secondary outcomes of the study include reduced anxiety symptoms, reduced loneliness, reduced rumination, improved social support, improved health-related quality of life, as well as improved quality-adjusted life years, and reduced healthcare service utilisation.

Design: While a randomised controlled trial would be ideal, such a study design was not possible in our current study context given pragmatic reasons. A quasi-experimental randomised controlled trial involving an (i) intervention arm and (ii) control arm will therefore be adopted. A targeted sample of 352 participants will be recruited (n = 176 per arm). To more rigorously account for potential differences in the sample characteristics of the two arms, propensity-score matching will be applied. After the pre-intervention or baseline (T0), assessments will be conducted at two follow-up time points: post-intervention (9-month post-baseline in the active control group, which is aligned with the median service duration in the intervention arm; T1), and 3-month post-intervention (12-month post-baseline in the active control group; T2).

Study setting

In Hong Kong, there is now at least one core district-based primary care centre at each of the 18 districts locally. Aside from the regular JoyAge intervention offered in the social care setting, the present study will focus on the effectiveness of the JoyAge intervention carried out at three main primary care centre sites in Hong Kong. Participants of the control group will be recruited from the community, comprising individuals who may be receiving services from non-JoyAge-partnered district-based primary care centres (that are not actively offering psychological interventions) or other services.

Power calculation for sample size

Taking a more conservative effect size reported in a meta-analysis examining the effects of psychotherapy for subclinical depression (using the lower confidence interval [CI] of CBT, yielding f = 0.14) (Cuijpers et al., 2014), a minimum sample size of 270 is required to achieve a power of 80% at the 0.05 alpha level. Further accounting for a potential 30% attrition given the setting in which this study is conducted, a total sample of 352 is targeted (n = 176 per arm).

Procedures and intervention: Experimental group

For those in the JoyAge partnered district-based primary care centres, participants identified to be showing some symptoms of depression (Patient Health Questionnaire-2-item ≥ 3) will be directed to the JoyAge team for a more in-depth assessment, involving the completion of a full Patient Health Questionnaire-9-item (PHQ-9). Those meeting the study inclusion criteria (see below) and have provided informed consent will be provided with interventions according to initial symptom severity, which are predominantly based on cognitive behavioural therapy (CBT) with a stepped-care element. Details are as follows:

• For those with mild symptoms (PHQ-9 = 5-9) without major risks: 4 weekly CBT-informed intervention sessions led by trained Emotional Support Assistants (ESA) (Peer Supporters with 80hrs mental health training + 15hrs intensive clinical training).
• For those with mild symptoms (PHQ-9 = 5-9) with major risk factors or moderate-to-moderately severe symptoms (PHQ-9 = 10-19): 6 weekly CBT-based intervention sessions led by trained clinical social workers, with 2 booster sessions as necessary. Individual CBT-based intervention will be provided to those unsuitable for group interventions.

Based on the initial trial of intervention, participants may be either "stepped-up" or "stepped-down" (or exit from service). Specifically:

- For all participants who were unable to achieve clinical remission following the first trial of intervention (PHQ-9 ≥ 5): a second trial of intervention will be provided with greater intensity (e.g., those receiving ESA-led interventions may be stepped up to worker-led interventions). Those needing additional support will be referred to a partnered General Practitioner (if PHQ-9=10-19) or referred to more intensive psychological or psychiatric services as needed.

*Definition of major risk factors: PHQ-9 item 9 (suicidal ideation) ≥ 2; moderate symptoms of generalised anxiety disorder (GAD-7 ≥ 10); high loneliness (UCLA 3-item ≥ 7); recent highly stressful or traumatic event (e.g., bereavement, domestic violence); complex high-risk condition (e.g., major family issues).

The recommended service period will be 6-9 months. All assessments will be conducted pre-intervention/baseline (T0), post-intervention (i.e., exit from service; T1), and 3-month post-intervention (T2).

Procedures and intervention: Active control group

Participants will be from community members who are not receiving services from the JoyAge-partnered district-based primary care centres. Upon identification of potential presentation of depressive symptoms, a trained research assistant will further verify their eligibility. Upon confirmation of their eligibility and informed consent provision, assessments will be conducted by a trained research assistant at baseline (T0), at 9-month follow-up (T1) [*aligned with the median service duration in the experimental group], and 12-month follow-up (T2). To ensure the baseline characteristics of the two groups are similar, propensity score matching will be applied, accounting for core demographics (e.g., sex, age) and initial symptom severity (e.g., mild / moderate / moderately severe).

Statistical methods

Sample characteristics will be examined and reported for both experimental arms at all time points. The primary analysis will follow an intention-to-treat (ITT) principle (i.e., all participants will be included as long as they were randomised, and based on their randomised allocation, regardless of the intervention they received or rate of completion).

To test the effects of the intervention on the primary and secondary outcomes of the present study, two approaches will be adopted:

1. Between-group differences will first be analysed using the generalised linear models. Core sociodemographics would be adjusted in these models. Unstandardised coefficients and effect sizes (Cohen's d) with 95% confidence intervals will be reported.
2. Linear mixed models will be applied to further confirm the results, which would account for the random effects at individual participant level.

Inclusion Criteria:

• Residing in Hong Kong;
• Presenting mild to moderately severe depressive symptoms (PHQ-9 = 5-19);
• Able to give informed consent to participate.

Exclusion Criteria:

• A known history of autism, intellectual disability, schizophrenia-spectrum disorder, bipolar disorder, Parkinson's disease, or dementia;
• Imminent suicidal risk;
• Difficulty in communication that would limit meaningful participation in psychological intervention.

IPD will not be shared due to participant confidentiality reasons.

Masking was not possible in this study.